This invention is related to the multiple access sharing of a single communication channel by many transmitters of digital information.
Code Division Multiple Access (CDMA) is a general technique used in telecommunication systems to allow multiple access (the sharing of a single communication channel by many transmitters) in a variety of telecommunications media. The media can be satellite channels, microwave and general radio channels or even fiber, cable and wire channels. CDMA channels generally are configured so that many transmitters radiate signals to a single hub station. This invention is generally concerned with the simplification of the process of transmission of digital signals from many remote terminals to a single hub station as shown in FIG. 1.
The "code division" in CDMA refers to the fact that in the prior art all of these CDMA channels rely on the use of a different binary code by each transmitter in the network in order to separate signals from different transmitters at the hub receiver. See for example Norman Abramson, Multiple Access Communications: Foundations for Emerging Technologies, (editor) IEEE Press, 1993. In the prior art the general understanding that multiple codes and code division are required for the effective operation of a CDMA channel is explicitly contained in many CDMA patents. See for example U.S. Pat. No. 4,455,651 (Baran), U.S. Pat. No. 4,841,527 (Raychaudhuri et al.), U.S. Pat. No. 5,136,612 (Bi), U.S. Pat. No. 5,151,919 (Dent), U.S. Pat. No. 5,218,619 (Dent), U.S. Pat. No. 5,235,612 (Stillwell et al.), and U.S. Pat. No. 5,305,348 (Izumi).
We have discovered that different codes in a CDMA system are not necessary in order to distinguish among different receivers in a given network. The separation of signals from different receivers has much more to do with the high bandwidth required in a CDMA network, rather than any properties of codes. If a CDMA system is built with the same code for all users in a channel using a high bandwidth, signals from different transmitters will still be separated at the receiver. This is the version of CDMA which is the basis of the claims in this patent. Our claims relate to certain key simplifications in the prior art made possible by elimination of multiple codes in CDMA data networks. For historical reasons we refer to our simplified version of CDMA which uses the same code for all transmitters as Spread ALOHA/CDMA(SA/CDMA).
The general misconception in the prior art dealing with the purpose of different codes in CDMA can be traced to the military origins of CDMA, where the purpose of the different codes used in military CDMA systems has much more to do with maintaining secrecy in communications than with multiple access. See for example Marvin K. Simon, Jim K. Omura, Robert A. Scholtz and Barry K. Levitt, Spread Spectrum Communications, Volumes 1,2 and 3, Computer Science Press, Rockville, Md.,1985.
The use of multiple codes in CDMA systems today results in major complications in the design of such systems. The complications flow from the fact that since each user in these systems has a separate code and since the code is used to separate one user from another, the hub station in a CDMA network must contain a different receiver for each user code in the network. Thus the hub stations are unreasonably complicated, and CDMA systems have in general been the subject of much criticism because of this complication. Furthermore many functions of a CDMA system are made more complex, and many subsidiary functions which could otherwise be used in the system are precluded because of the fact that the hub receiver does not know the code which is used in each signal being received. If all transmitters use the same signal many more options for the design of the system become available.
The transmission of a signal from a single user in a multiple access CDMA channel can be described in terms of a complex signal representation as shown in FIG. 2. The general signal modulation process is represented as the multiplication of the data input signal a (t) by a modulated carrier, b (t). The resulting transmitted signal is EQU c(t)=a(t)b(t) (1)
If we require that b (t) the modulated carrier for this user satisfies EQU .vertline.b(t).vertline..sup.2 =1 (2)
then the fundamental operation of a receiver is to multiply the received signal plus noise, c(t)+n(t) , by the complex conjugate of the modulated carrier, b*(t) , in order to recover the original information in a (t). EQU [c(t)+n(t)]b*(t)=a(t)+n(t)b*(t) (3)
This representation is general enough to include conventional forms of modulation, when b (t) is a narrowband carrier. But in the case of CDMA, the modulating carrier, b(t), is a different wideband carrier for each possible terminal in the network. This multiplicity of possible transmitter carriers is reflected in a multiplicity of receivers required at a CDMA base station in order to demodulate the received signal. Clearly if the total number of potential data terminals in the network is much larger than the number of terminals active at any given time this requirement can introduce considerable complexity into the design of a CDMA multiple access system.